Expandable endoprosthesis devices (i.e., stents) are adapted to be implanted into a patient's body lumen (e.g., a blood vessel) to maintain the patency thereof. There are two primary categories of stents, which are balloon expandable stents and self-expanding stents. Balloon expandable stents are made from materials (e.g. stainless steel, cobalt chromium alloys, etc.) and/or geometries that are plastically deformed by via the application of external forces against the balloon expandable stent (e.g., the expansion of the balloon against the stent). Once deformed, a balloon expandable stent maintains its plastically deformed state when the force is removed. Self-Expanding stents are made from materials (e.g. Nitinol, Algiloy, other shape memory alloys) and/or geometries (e.g., braided members, etc.) that are elastically deformed by external forces, but elastically return to their pre-deformed shape when the external force is removed.
Stents are particularly useful retaining vessel patency in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA) or percutaneous transluminal angioplasty (PTA), or a stenosis has been removed by atherectomy or other means. Deploying a stent improves the results of the aforementioned procedures and reduces the possibility of restenosis.
In general, balloon expandable stent systems are the preferred method for treatment of intravascular stenosis. This is primarily driven by ease of use resulting from improved system flexibility, lower profile, better placement accuracy, etc. and inherent stent implant performance. preferred method of treatment due to the location of the stent implant. Specifically when stents are implanted in a location of the body (carotid artery, iliac artery, superficial femoral artery, etc.) where an external force may be applied to the implanted stent, it is necessary that, once the external force is removed, the stent return to its un-deformed (prior) shape in order to maintain lumen patency.
Stents are generally cylindrically shaped devices that function to hold open, and sometimes expand, a segment of a blood vessel or other arterial lumen (e.g., coronary or carotid arteries). Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position.
Stent delivery systems for balloon expandable stents typically include a tubular body (e.g., a catheter, sheath, etc.) having an inner guidewire lumen and a balloon extending over a distal portion of the tubular body. A balloon inflation lumen runs the length of the tubular body. The stent is compressed over the collapsed balloon. To help secure the stent in a compressed state about the balloon, it is often necessary to provide a retaining mechanism such as retaining structures under the balloon or over the stent ends (e.g., sleeves at each end of the stent). Such retaining mechanisms increase catheter bulk, stiffness and profile and decrease system trackability. Other retaining mechanisms known in the art include heat-treating the balloon to conform to the stent. U.S. Pat. Nos. 6,159,227, 6,096,056, 6,203,558 and 6,478,814 disclose various retaining mechanisms and are incorporated by reference herein in their entireties.
Stent delivery systems for implanting self-expandable stents typically include a tubular body (e.g., catheter, sheath, etc.) having an inner guidewire lumen, a compressed or collapsed stent mounted near the distal end of the tubular body, and an outer restraining sheath initially placed over the compressed stent prior to stent deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved proximally to uncover the compressed stent. This allows the stent to move to its expanded condition.
Some self-expanding delivery systems utilize a push-pull technique in which the outer sheath is retracted while the inner tubular body is pushed forward. Still other systems use an actuating wire that is attached to the outer sheath. When the actuating wire is pulled to retract the outer sheath and deploy the stent, the inner tubular body remains stationary to prevent the stent from moving axially within the body vessel.
An example of a prior art self-expanding stent 5 is illustrated in FIGS. 1 and 2. FIG. 1 is a longitudinal elevation of the prior art self-expanding stent 5 in a compressed or non-deployed configuration as the stent 5 would appear within a retaining sheath 10 of a delivery system 15. FIG. 2 is a longitudinal elevation of the prior art self-expanding stent 5 of FIG. 1 in a fully deployed or expanded state as the stent 5 would appear within a body lumen 20 of a patient after the retaining sheath 10 of the delivery system 15 has been withdrawn to allow the stent 5 to deploy.
As can be understood from FIGS. 1 and 2, the self-expanding stent 5 has a cylindrical body 25, which radially expands substantially in diameter from the non-deployed state depicted in FIG. 1 to the deployed state depicted in FIG. 2. As indicated in FIGS. 1 and 2, the stent body 25 is formed from a plurality of adjacent inter-connected struts 30 arranged in a cellular pattern to form the outer circumferential surface of the stent 5. The body 25 is radially expandable. The cellular pattern of the inter-connected struts 30 forms cells 35. Each cell 35 joins with radially circumferentially adjacent cells 35 via cell interconnections 40 to form radially circumferentially continuous cell rings 45. With the exception of the most distal and proximal cell rings 50, 55, each cell ring 45 is sandwiched between, and joined to, its two longitudinally adjacent cell rings 45 via ring interconnections 60 to form a longitudinally continuous stent 5. The cell interconnections 40 align longitudinally and radially along the stent 5 with each other, and the ring interconnections 60 align longitudinally and radially along the stent 5 with each other. However, the cell and ring interconnections 40, 60 are longitudinally and radially offset from each other.
The stent 5 is retained in its compressed or non-deployed state, as depicted in FIG. 1, by a retaining sleeve or sheath 10 extending about the stent 5 when the stent 5 is being negotiated through a body lumen 20 of a patient. Upon reaching the stent implant location wherein plaque 65 is present, the sleeve 10 is withdrawn from about the stent 5, thereby freeing the stent 5 to expand to its fully expanded or deployed state, as depicted in FIG. 2. For further discussion regarding the exemplary self-expanding stent 5 of FIGS. 1 and 2, reference is made to U.S. Pat. No. 6,814,746, which issued Nov. 9, 2004, and is hereby incorporated by reference in its entirety.
Problems have been associated with the aforementioned delivery systems. For example, systems that rely on a push-pull design can experience movement of the collapsed stent within the body vessel when the inner tubular body is pushed forward. This can lead to inaccurate positioning and, in some instances, possible perforation of the vessel wall by a protruding end of the stent. Also, the thickness of the outer sheath adds to the delivery system profile and makes the system less trackable due to added tubular body stiffness.
Systems that utilize an actuating wire or full-length sheath design tend to move to shortcut the curvature of the anatomy of the patient due to increased stiffness of such systems. As the wire or sheath is pulled proximal, compression in the delivery system can cause the system to change position in the vasculature, leading to inaccurate stent placement.
Stent deployment systems that employ a restraining sheath have diameters that are large in comparison to stent delivery systems without a sheath. This is problematic when trying to deploy the stent in a body lumen having a reduced diameter. Sheaths and wires also add stiffness to the catheter, which inhibits tracking through the vasculature.
During deployment of a self-expanding stent using a proximally retractable sheath, the stent has a tendency to jump distally due to the expansion force from the distal end of the partially expanded stent acting against the restraint of the smaller diameter sheath end. This causes the stent to be displaced axially (distally) and contributes to inaccurate placement. Various restraint systems/methods have been contemplated to eliminate this axial movement. Examples include compressing the stent radially against friction surfaces, interlocking surfaces that project through stent openings, or tethers and wires that hold back the stent. Examples of such systems/methods are found in the following U.S. Pat. Nos. 6,582,460; 6,530,947; 6,517,547; 6,425,898; 6,254,609; 5,709,703; 5,607,466; 6,251,132; 6,350,277; 6,120,522; 6,814,746 and 6,576,006. These patents are incorporated herein by reference in their entireties.
To reduce the risk of distal embolization and possible neurological impairment from plaque disruption, distal embolization protection has become an essential component of stent deployment techniques in carotid arteries and bypass grafts. Current carotid artery stent deployment techniques require the following steps: placement of a distal embolic filter or proximal flow control protection system; pre-dilation of the stenosis (optional); deployment of the stent; post dilation of the deployed stent as needed; and removal of the embolic protection system. Such techniques are unnecessarily complicated because they require the insertion and withdrawal of multiple devices, increase case time, and increase the risk of stent misplacement and dislodgement by devices passing through the stent. In addition each catheter exchange introduces the possibility of plaque disruption and embolization, which could be missed by embolic protection systems, potentially increasing the risk for stroke, or flow restrictions in distal arteries. One example of an additional catheter exchange is the need to remove the stent delivery system following self-expanding stent deployment and the introduction of a separate balloon catheter to dilate the stent to its final diameter.
There have been attempts in the art to place a balloon on the same catheter as the self-expanding stent to eliminate a catheter exchange. U.S. Pat. No. 5,360,401, which issued Nov. 1, 1994 and is incorporated herein by reference in its entirety, describes a stent placed over a balloon with a protective pull back sheath over the stent. U.S. Pat. No. 5,843,090, which issued Dec. 1, 1998 and is incorporated herein by reference in its entirety, describes an uncovered balloon with a sheath bonded to it on the underside. The sheath restrains a stent placed under it in the compressed state. Both the balloon and sheath are retracted together to expose the self-expanding stent during deployment. After deployment the balloon and sheath are advanced through the stent and the stent post dilated. Although both of these concepts eliminate the exchange of the stent delivery system for a post dilatation balloon, both still utilize a sheath and require the pull back step. Furthermore, both have increased materials that add to the delivery profile and contribute to excess stiffness and poor trackability through the vasculature.
There is a need in the art for an improved expandable stent and a system for, and method of, deploying such a stent that offers improved ease of use, increased stent placement accuracy, lower delivered profile, and better trackability through the vasculature. There is also need for an improved expandable stent and a system for, and method of, deploying such a stent that offers a reduced number of catheter exchanges, thereby reducing the risk of plaque embolization, distal flow obstruction, and stroke.